Scavenging behavior and interspecific competition decrease offspring fitness of the entomopathogenic nematode Steinernema feltiae

Entomopathogenic nematodes (EPNs) are well-studied biocontrol agents of soil-dwelling arthropod pests. The insecticidal efficiency of EPNs is modulated by food web dynamics. EPNs can reproduce in freeze-killed insect larvae, even in competition with free-living bacterivorous nematodes (FLBNs) in the genus Oscheius. The objective of this study was to assess the efficiency of EPNs as scavengers when competing with free-living saprophagous nematodes and fungi, and to determine the possible impact on subsequent EPN offspring fitness. Live and freeze-killed larvae of Galleria mellonella were used to evaluate the reproduction rate and progeny fitness of two EPN species, Heterorhabditis bacteriophora and Steinernema feltiae, applied individually or combined with the FLBN species Oscheius onirici or Pristionchus maupasi, or Aspergillus flavus, an opportunistic saprophytic fungus. We hypothesized that (1) EPN scavenging behaviors previously observed (for H. megidis and S. kraussei) apply to other EPN species, (2) infective juveniles (IJs) emerging from freeze-killed larvae will display reduced pathogenicity and reproduction, and (3) fitness reduction will be amplified by exposure to other organisms competing for the resources. The reproduction rate of S. feltiae was lower in freeze-killed larvae than in larvae infected and killed by the nematode, whereas H. bacteriophora failed to reproduce as a scavenger. The S. feltiae F1 IJs that emerged from freeze-killed larvae exhibited lower pathogenicity rates than IJs resulting from entomopathogenic activity, and also lower reproductive rates if they experienced high FLBN competitive pressure during development. This study illustrates that scavenging is a suboptimal alternative pathway for EPNs, especially in the face of scavenger competition, even though it provides a means for some EPN species to complete their life-cycle.